The present invention relates to a method for forming connection protrusions called xe2x80x9cbumpsxe2x80x9d by mounting electrically conductive balls represented by solder balls or gold spheres onto a package such as a BGA (ball grid array) package, a CSP (chip size package or chip scale package), etc., of the type (hereinafter simply referred to as xe2x80x9cpackagexe2x80x9d) in which solder balls are used as a bonding material for bonding the package to a mounting substrate, among various LSI packages.
In a recent LSI package substrate, or the like, used in an LSI having a large number of input/output terminals, there is employed a structure in which a plurality of electrical terminals arranged in the form of a grid or a zigzag grid are provided on a lower surface of the LSI package substrate, and the electrical terminals are connected by bumps to circuit electrodes of circuit boards corresponding to the electrical terminals.
A solder bump formation method is disclosed in JP-A-8-115916, JP-A-9-063737, etc. Those publications disclose: a technique for forming solder bumps by vacuum-sucking solder balls by means of an adsorbing jig, dipping the solder ball into a flux liquid tank to thereby supply flux to the solder balls, transferring the solder balls onto pads (connection terminals) formed on an LSI package substrate, and heating (reflowing) an electronic circuit substrate containing the solder balls tackily held (temporarily fixed) by tacking force of the flux; an apparatus for mounting solder balls onto an LSI package substrate. The heating may be carried out by using a reflow furnace available on the market.
The conventional method, however, has the following problems.
A flexible printed circuit board such as a flexible tape has been used widely as the LSI package substrate recently. FIG. 12 shows an example of an LSI package using such a flexible printed circuit board. In FIG. 12, the reference numeral 3 designates a flexible printed circuit board; 15, solder bumps; 4, pads on which the solder bumps 15 are formed; 11, a semiconductor chip; 19, leads for connecting the pads 4 to the semiconductor chip 11; 18, an encapsulation resin for fixing the semiconductor chip 11 to the flexible printed circuit board 3; and 17, a quadrate reinforcement plate.
In the case of an LSI package using the aforementioned flexible printed circuit board, the LSI package substrate is trimmed into small pieces shaped like strips in advance before formation of bumps to make it possible to use a conventional manufacturing apparatus for a rigid printed circuit board. The LSI package substrate is further trimmed into minimum pieces in the final step of LSI packaging after bump formation. Thus, two steps are required for trimming. Accordingly, there arises a problem that the cost for bump formation is high.
Further, in order to put the strips of LSI package substrate into a ball mounter or a heating reflow furnace, which is an apparatus for mounting solder balls onto the LSI package substrate, it is necessary to make the handling easy by fixing the package substrate, which is low in rigidity and hard to handle, to an exclusive-use carrying jig which is high in rigidity. High dimensional accuracy is required in the exclusive-use carrying jig, so that the fixing of the LSI package substrate to the exclusive-use carrying jig is mainly performed by hand work. As described above, conventionally, not only a number of expensive exclusive-use carrying jigs are prepared in accordance with the number of products but also strips of LSI package substrate are fixed to the exclusive-use carrying jigs. Accordingly, there arises a problem that the cost for bump formation is high.
To solve the aforementioned problems, it is thought of that the solder ball mounter 14 and the reflow furnace are remodeled to make it possible to supply a tape-like LSI package substrate continuously, and then the solder ball mounter and the reflow furnace are integrated with each other, so that the LSI package substrate after completion of mounting of solder balls is put into the reflow furnace directly to thereby eliminate the exclusive-use carrying jigs and the substrate trimming steps for bump formation.
In this method, however, when a trouble occurs in he solder ball mounter for mounting solder balls so that the apparatus stops, it is difficult to prevent the damage of the LSI package substrate exposed to a high temperature in an in-process state in the reflow furnace. Further, though it is general that the time required for mounting solder balls varies in accordance with the presence/absence of a retry operation, or the like, the change of the heating time leads to the change of the total amount of heat energy received by semiconductors connected to the solder balls and the LSI package substrate and, accordingly, it causes failures in bump formation. Accordingly, the change of the heating time in bump formation is not permitted, so that the integration of the solder ball mounter and the reflow furnace is unreasonable from the two apparatuses"" characters and cannot be achieved at present.
The present invention is intended to realize a bump formation method in which the aforementioned problems are solved and the exclusive-use carrying jigs and the substrate trimming steps for bump formation are made unnecessary with respect to a tape-like LSI package substrate.
An object of the present invention is to provide a bump formation method in which bumps can be formed on a semiconductor substrate by a simple structure and by a simple assembling means.
Another object of the present invention is to provide a bump formation method which is excellent in reliability on formation of bumps on a semiconductor substrate.
A further object of the present invention is to provide a bump formation method in which bumps can be formed on a semiconductor substrate inexpensively.
In order to achieve the aforementioned objects, it is designed that a tape-like LSI package substrate is fed from a reel into a solder ball mounter, and the substrate is temporarily wound up onto the reel after solder balls have been mounted on the substrate. The tackiness of a fixing liquid such as flux, or the like, used for mounting the solder balls onto the substrate is selected to be larger than a value of the product of the mass of the solder balls and the acceleration received by the solder balls during carrying the substrate so that the solder balls mounted and temporarily fixed on the substrate are prevented from dropping. Further, when the tape-like LSI package substrate is wound up onto the reel, a spacer is involved in the windings of the substrate to prevent mechanical contact so that the solder balls mounted and temporarily fixed to the substrate are not displaced because of the mechanical contact.
According to the present invention, the formation of bumps on a tape-like flexible printed circuit board widely used as an LSI package substrate can be achieved inexpensively without using any trimming step for forming bumps and without using any exclusive-use carrying jig. In addition, remodeling of the general-use solder ball mounter and the general-use reflow furnace can be minimized.